Hybrid gate stack integration for stacked vertical transport field-effect transistors

ABSTRACT

A method of forming a semiconductor structure includes forming one or more vertical fins each including a first semiconductor layer providing a vertical transport channel for a lower vertical transport field-effect transistor (VTFET) of a stacked VTFET structure, an isolation layer over the first semiconductor layer, and a second semiconductor layer over the isolation layer providing a vertical transport channel for an upper VTFET of the stacked VTFET structure. The method also includes forming a first gate stack including a first gate dielectric layer and a first gate conductor layer surrounding a portion of the first semiconductor layer of the vertical fins. The method further includes forming a second gate stack including a second gate dielectric layer and a second gate conductor layer surrounding a portion of the second semiconductor layer of the vertical fins. The first gate conductor layer and the second gate conductor layer are the same material.

BACKGROUND

The present application relates to semiconductors, and morespecifically, to techniques for forming semiconductor structures.Semiconductors and integrated circuit chips have become ubiquitouswithin many products, particularly as they continue to decrease in costand size. There is a continued desire to reduce the size of structuralfeatures and/or to provide a greater amount of structural features for agiven chip size. Miniaturization, in general, allows for increasedperformance at lower power levels and lower cost. Present technology isat or approaching atomic level scaling of certain micro-devices such aslogic gates, field-effect transistors (FETs), and capacitors.

SUMMARY

Embodiments of the invention provide techniques for hybrid gate stackintegration for stacked vertical transport field-effect transistorstructures.

In one embodiment, a method of forming a semiconductor structurecomprises forming one or more vertical fins each comprising a firstsemiconductor layer providing a vertical transport channel for a lowervertical transport field-effect transistor of a stacked verticaltransport field-effect transistor structure, an isolation layer over thefirst semiconductor layer, and a second semiconductor layer over theisolation layer providing a vertical transport channel for an uppervertical transport field-effect transistor of the stacked verticaltransport field-effect transistor structure. The method also comprisesforming a first gate stack comprising a first gate dielectric layer anda first gate conductor layer surrounding a portion of the firstsemiconductor layer of each of the one or more vertical fins. The methodfurther comprises forming a second gate stack comprising a second gatedielectric layer and a second gate conductor layer surrounding a portionof the second semiconductor layer of each of the one or more verticalfins. The first gate conductor layer and the second gate conductor layercomprise a same material.

In another embodiment, a semiconductor structure comprises one or morevertical fins each comprising a first semiconductor layer providing avertical transport channel for a lower vertical transport field-effecttransistor of a stacked vertical transport field-effect transistorstructure, an isolation layer over the first semiconductor layer, and asecond semiconductor layer over the isolation layer providing a verticaltransport channel for an upper vertical transport field-effecttransistor of the stacked vertical transport field-effect transistorstructure. The semiconductor structure also comprises a first gate stackcomprising a first gate dielectric layer and a first gate conductorlayer surrounding a portion of the first semiconductor layer of each ofthe one or more vertical fins. The semiconductor structure furthercomprises a second gate stack comprising a second gate dielectric layerand a second gate conductor layer surrounding a portion of the secondsemiconductor layer of each of the one or more vertical fins. The firstgate conductor layer and the second gate conductor layer comprise a samematerial.

In another embodiment, an integrated circuit comprises a stackedvertical transport field-effect transistor structure. The stackedvertical transport field-effect transistor structure comprises one ormore vertical fins each comprising a first semiconductor layer providinga vertical transport channel for a lower vertical transport field-effecttransistor of the stacked vertical transport field-effect transistorstructure, an isolation layer over the first semiconductor layer, and asecond semiconductor layer over the isolation layer providing a verticaltransport channel for an upper vertical transport field-effecttransistor of the stacked vertical transport field-effect transistorstructure. The stacked vertical transport field-effect transistorstructure also comprises a first gate stack comprising a first gatedielectric layer and a first gate conductor layer surrounding a portionof the first semiconductor layer of each of the one or more verticalfins. The stacked vertical transport field-effect transistor structurefurther comprises a second gate stack comprising a second gatedielectric layer and a second gate conductor layer surrounding a portionof the second semiconductor layer of each of the one or more verticalfins. The first gate conductor layer and the second gate conductor layercomprise a same material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-sectional view of a semiconductor on insulator onsemiconductor substrate, according to an embodiment of the invention.

FIG. 2 depicts a cross-sectional view of the FIG. 1 structure followingformation of top vertical fins, according to an embodiment of theinvention.

FIG. 3 depicts a cross-sectional view of the FIG. 2 structure followingformation of a liner on sidewalls of the top vertical fins, according toan embodiment of the invention.

FIG. 4 depicts a cross-sectional view of the FIG. 3 structure followingformation of bottom vertical fins and formation of an additional liner,according to an embodiment of the invention.

FIG. 5 depicts a cross-sectional view of the FIG. 4 structure followingrecess and trimming of portions of the substrate, according to anembodiment of the invention.

FIG. 6 depicts a cross-sectional view of the FIG. 5 structure followingformation of bottom source/drain regions for the lower verticaltransport field-effect transistors and following formation of shallowtrench isolation regions surrounding the bottom source/drain regions,according to an embodiment of the invention.

FIG. 7 depicts a cross-sectional view of the FIG. 6 structure followingformation of a bottom spacer and deposition of gate stack material,according to an embodiment of the invention.

FIG. 8 depicts a cross-sectional view of the FIG. 7 structure followinga gate cut, according to an embodiment of the invention.

FIG. 9 depicts a cross-sectional view of the FIG. 8 structure followingformation of an interlayer dielectric and recess of the gate stack,according to an embodiment of the invention.

FIG. 10 depicts a cross-sectional view of the FIG. 9 structure followingformation of a top spacer and top source/drain regions for the lowervertical transport field-effect transistors, according to an embodimentof the invention.

FIG. 11 depicts a cross-sectional view of the FIG. 10 structurefollowing formation of a sacrificial oxide layer and followingpatterning of an organic planarization layer, according to an embodimentof the invention.

FIG. 12 depicts a cross-sectional view of the FIG. 11 structurefollowing removal of exposed portions of the sacrificial oxide layer,removal of the organic planarization layer and formation of an isolatinglayer, according to an embodiment of the invention.

FIG. 13 depicts a cross-sectional view of the FIG. 12 structurefollowing removal of the protection liner and formation of an oxidelayer, according to an embodiment of the invention.

FIG. 14 depicts a cross-sectional view of the FIG. 13 structurefollowing formation of an additional protection liner, according to anembodiment of the invention.

FIG. 15 depicts a cross-sectional view of the FIG. 14 structurefollowing removal of the oxide layer and formation of bottomsource/drain regions for the upper vertical transport field-effecttransistors, according to an embodiment of the invention.

FIG. 16 depicts a cross-sectional view of the FIG. 15 structurefollowing formation of a sacrificial oxide layer, according to anembodiment of the invention.

FIG. 17A depicts a first cross-sectional view of contacts for a NANDlogic gate, according to an embodiment of the invention.

FIG. 17B depicts a second cross-sectional view of contacts for a NANDlogic gate, according to an embodiment of the invention.

FIG. 17C depicts a third cross-sectional view of contacts for a NANDlogic gate, according to an embodiment of the invention.

FIG. 17D depicts a fourth cross-sectional view of contacts for a NANDlogic gate, according to an embodiment of the invention.

FIG. 17E depicts a fifth cross-sectional view of contacts for a NANDlogic gate, according to an embodiment of the invention.

FIG. 18 depicts a cross-sectional view of the FIG. 16 structure afterpatterning of the sacrificial oxide layer, removal of the protectionliner, and following formation of a bottom spacer, according to anembodiment of the invention.

FIG. 19 depicts a cross-sectional view of the FIG. 18 structurefollowing formation of the gate stack for the upper vertical transportfield-effect transistors, an interlayer dielectric layer, and a topspacer, according to an embodiment of the invention.

FIG. 20 depicts a cross-sectional view of the FIG. 19 structurefollowing removal of the hard mask layer and dopant drive-in for a topjunction of the upper vertical transport field-effect transistors,according to an embodiment of the invention.

FIG. 21 depicts a cross-sectional view of the FIG. 20 structurefollowing formation of top source/drain regions for the upper verticaltransport field-effect transistors, according to an embodiment of theinvention.

DETAILED DESCRIPTION

Illustrative embodiments of the invention may be described herein in thecontext of illustrative methods for hybrid gate stack integration instacked vertical transport field-effect transistors, along withillustrative apparatus, systems and devices formed using such methods.However, it is to be understood that embodiments of the invention arenot limited to the illustrative methods, apparatus, systems and devicesbut instead are more broadly applicable to other suitable methods,apparatus, systems and devices.

A field-effect transistor (FET) is a transistor having a source, a gate,and a drain, and having action that depends on the flow of carriers(electrons or holes) along a channel that runs between the source anddrain. Current through the channel between the source and drain may becontrolled by a transverse electric field under the gate.

FETs are widely used for switching, amplification, filtering, and othertasks. FETs include metal-oxide-semiconductor (MOS) FETs (MOSFETs).Complementary MOS (CMOS) devices are widely used, where both n-type andp-type transistors (nFET and pFET) are used to fabricate logic and othercircuitry. Source and drain regions of a FET are typically formed byadding dopants to target regions of a semiconductor body on either sideof a channel, with the gate being formed above the channel. The gateincludes a gate dielectric over the channel and a gate conductor overthe gate dielectric. The gate dielectric is an insulator material thatprevents large leakage current from flowing into the channel whenvoltage is applied to the gate conductor while allowing applied gatevoltage to produce a transverse electric field in the channel.

Increasing demand for high density and performance in integrated circuitdevices requires development of new structural and design features,including shrinking gate lengths and other reductions in size or scalingof devices. Continued scaling, however, is reaching limits ofconventional fabrication techniques.

Stacking FETs in a vertical direction gives an additional dimension forCMOS area scaling. It is difficult, however, to stack planar FETs.Vertical transport FETs (VTFETs) are being pursued as viable CMOSarchitectures for scaling to 7 nanometers (nm) and beyond. VTFETsprovide the opportunity for further device scaling compared with otherdevice architectures. VTFETs have various potential advantages overother conventional structures such as fin field-effect transistors(FinFETs). Such advantages may include improvements in density,performance, power consumption, and integration. VTFETs may furtherprovide advantages in stacking FETs.

Illustrative embodiments provide techniques for hybrid gate stackintegration in stacked VTFETs, which enables the use of a same gateconductor material (e.g., a same work function metal (WFM) such astitanium nitride (TiN)) used for both the upper and lower VTFETs in thestacked VTFET structure. The gate conductor material, or WFM material,plays an important role in directed stacked VTFET integration.Illustrative embodiments enable the use of the same gate conductormaterial (e.g., a WFM such as TiN) for upper and lower VTFETs in astacked VTFET structure by using a rapid thermal anneal (RTA) for thelower VTFETs (e.g., which provide nFET devices in some embodiments) andusing a laser-only anneal for the upper VTFETs (e.g., which provide pFETdevices in some embodiments). The gate stack for the lower VTFETs (e.g.,the bottom nFET gate stack) is formed using a gate-first process, whilethe gate stack for the upper VTFETs (e.g., the top pFET gate stack) isformed using a gate-last process.

Embodiments provide techniques for forming stacked VTFET structures,where the lower VTFETs or bottom-tier nFET devices use a gate-firstprocessing flow and the upper VTFETs or top-tier pFET devices use agate-last processing flow. The gate-first flow uses an annealed gateconductor material (e.g., an annealed WFM such as annealed TiN) in thegate stack for the bottom-tier nFET devices, and the gate-last flow usesthe same gate conductor material un-annealed (e.g., an un-annealed WFMsuch as un-annealed TiN). Gate dielectric deposition and reliabilityanneal processes happen twice, once for the bottom-tier nFET devices andonce for the top-tier pFET devices.

Illustrative processes for hybrid gate stack integration in stackedVTFET structures that enable use of a same gate conductor material forboth the upper and lower VTFETs of the stacked VTFET structures will nowbe described with respect to FIGS. 1-21.

FIG. 1 shows a cross-sectional view 100 of a bulk substrate 102, aninsulator layer 104 formed over the bulk substrate 102, and asemiconductor layer 106 formed over the insulator layer 104. Thesemiconductor layer 106 and insulator 104 may form a thin buried oxide(BOX) silicon-on-insulator (SOI).

The bulk substrate 102 and semiconductor layer 106 may be formed of anysuitable semiconductor structure, including various silicon-containingmaterials including but not limited to silicon (Si), silicon germanium(SiGe), silicon germanium carbide (SiGeC), silicon carbide (SiC) andmulti-layers thereof. Although silicon is the predominantly usedsemiconductor material in wafer fabrication, alternative semiconductormaterials can be employed as additional layers, such as, but not limitedto, germanium (Ge), gallium arsenide (GaAs), gallium nitride (GaN),SiGe, cadmium telluride (CdTe), zinc selenide (ZnSe), etc.

The bulk substrate 102 and semiconductor layer 106 may have the same ordifferent crystalline orientations, depending on the types of VTFETdevices that are to be formed from the FIG. 1 structure. In someembodiments, for example, the bulk substrate 102 and semiconductor layer106 have different crystalline orientations for formation of nFET andpFET devices. For example, a first crystalline orientation (110) may beused for formation of nFET devices and a second crystalline orientation(100) may be used for formation of pFET devices.

For clarity of illustration, FIGS. 1-21 are shown and described withrespect to forming a stacked VTFET structure with just two VTFETsvertically stacked (e.g., in direction Y-Y′). In other embodiments,however, a stacked VTFET structure may include three or more VTFETsvertically stacked. Further, while FIGS. 1-21 are described with respectto stacking a VTFET of one type on top of a VTFET of another type (e.g.,where the upper VTFET is a pFET device and the lower VTFET is an nFETdevice), embodiments are not so limited. For example, the upper andlower VTFETs may both be nFET devices or pFET devices. Further, thestacked VTFETs may include any number of nFET devices formed over anynumber of pFET devices as desired for a particular application.

The horizontal thickness or width (in direction X-X′) of the FIG. 1structure may vary, such as based on the number of fins that are to beformed therefrom as described in further detail below. The verticalthickness or height (in direction Y-Y′) of the bulk substrate 102 may bein the range of 200 micrometers (μm) to 300 μm, and the verticalthickness or height (in direction Y-Y′) of the semiconductor layer 106may be in the range of 30 nm to 60 nm.

The insulator layer 104 may be formed of silicon dioxide (SiO₂) or anyother suitable dielectric material that provides for N-P isolation. Theinsulator layer 104 may have a height or vertical thickness (indirection Y-Y′) in the range of 10 nm to 30 nm.

FIG. 2 shows a cross-sectional view 200 of the FIG. 1 structurefollowing formation of a top portion of two vertical fins 201 for “top”transistors in stacked VTFET structures formed from the semiconductorlayer 106 and insulator layer 104. The top portion of the vertical fins201 may be formed using sidewall image transfer (SIT) or other suitabletechniques such as lithography and etching including reactive-ionetching (ME), etc. As shown, a hard mask layer (HM) 108 is patternedover the top surface of the semiconductor layer 106. Each of thevertical fins 201 may have a width or horizontal thickness (in directionX-X′) in the range of 6 nm to 10 nm.

The HM 108 may be formed of a nitride such as SiN, although othersuitable materials may be used. The HM 108, in some embodiments, may beformed as a multi-layer, such as a multi-layer of two layers including anitride and oxide (e.g., SiN and silicon dioxide (SiO₂)), a multi-layerof three layers including one or more nitride and one or more oxidelayers (e.g., SiN/SiO₂/SiN, SiO₂/SiN/SiO₂), etc. The HM 108 may have aheight or vertical thickness (in direction Y-Y′) in the range of 10 nmto 100 nm.

Although FIG. 2 shows two vertical fins 201 formed from the FIG. 1structure for clarity of illustration, it should be appreciated thatmore or fewer vertical fins may be formed from the FIG. 1 structure toform desired numbers of stacked VTFET structures.

FIG. 3 shows a cross-sectional view 300 of the FIG. 2 structurefollowing formation of a liner 110 to protect the top vertical fins 201during downstream processing described in further detail below. Theliner 110 may be formed from a very hard material, such as a high-kdielectric material such as hafnium oxide (HfO₂), high-k/SiNmultilayers, etc. The liner 110 may be formed via atomic layerdeposition (ALD). The liner 110 may have a thickness (in direction X-X′)in the range of 3 nm to 6 nm.

FIG. 4 shows a cross-sectional view 400 of the FIG. 3 structurefollowing extending the vertical fins 201 into the bulk substrate 102.Thus, the vertical fins 201 have “top” portions or top fins formed fromthe semiconductor layer 106 and “bottom” portions or bottom fins formedfrom the bulk substrate 102. The vertical fins 201 may be extended intothe bulk substrate 102 through additional etching (e.g., RIE). Thebottom portions of the vertical fins 201 may have a height or verticalthickness (in direction Y-Y′) in the range of 5 nm to 8 nm. In someembodiments, the bottom fin critical dimension (e.g., the width indirection X-X′) may be trimmed at this point or at the point of gateprocessing as described in further detail below.

FIG. 4 also shows an additional liner 112 that is formed on sidewalls ofthe vertical fins 201 after the vertical fins 201 are extended into thebulk substrate 102. The liner 112 is a protective liner, which may beformed of silicon boron carbide nitride (SiBCN). The liner 112 may beformed using selective ALD. The liner 112 may have a thickness (indirection X-X′) in the range of 2 nm to 4 nm.

In the description below, it is assumed that the top portions of thevertical fins 201 are used to form pFET devices and that the bottomportions of the vertical fins 201 are used to form nFET devices. Itshould be appreciated, however, that in other embodiments this may bereversed. Also, it is possible for both the bottom and top portions ofone or more of the vertical fins 201 to be used for forming a same typeof device (e.g., both nFETs, both pFETs). Various other combinations arepossible.

FIG. 5 shows a cross-sectional view 500 of the FIG. 4 structurefollowing additional recess of the bulk substrate 102 and followingtrimming of the recessed portion of the bulk substrate 102. The bulksubstrate 102 may be recessed below a bottom of the liner 112 to a depthin the range of 10 nm to 20 nm. The bulk substrate 102 may be recessedusing various etching processes, including RIE. The additional recess ofthe bulk substrate 102 provides room for growth of a bottom epitaxiallayer for a bottom source/drain region for the bottom VTFETs formed fromthe bottom portions of the vertical fins 201. Though the additionalrecess presents some risk as the vertical fins 201 are made even taller,there are benefits such as moving the bottom junction closer to the gateof the bottom VTFETs and in not requiring high temperature processes topush in dopants for the bottom junction.

After the additional recess of the bulk substrate 102, the recessedportion of the bulk substrate 102 may be trimmed (in direction X-X′) asillustrated. The depth of the trim (in direction X-X′) may be such thatthe recessed portion of the bulk substrate 102 has a same width orhorizontal thickness (in direction X-X′) as the bottom portion of thevertical fins 201. The recessed portion of the bulk substrate 102 may betrimmed using selective ME.

FIG. 6 shows a cross-sectional view 600 of the FIG. 5 structurefollowing formation of bottom source/drain regions 114 for the bottom orlower VTFETs in the stacked VTFET structure provided by each of thevertical fins 201, patterning of the bottom source/drain regions 114(e.g., using lithography and etching, which removes the liner 112), andformation of shallow trench isolation (STI) regions 116 surrounding thebottom source/drain regions 114.

The bottom source/drain regions 114 may have a height or verticalthickness (in direction Y-Y′) in the range of 15 to 30 nm. The bottomsource/drain regions 114 may have a width or horizontal thickness (indirection X-X′) in the range of 40 to 60 nm.

The bottom source/drain regions 114 may be formed, for example, byimplantation of a suitable dopant, such as using ion implantation, gasphase doping, plasma doping, plasma immersion ion implantation, clusterdoping, infusion doping, liquid phase doping, solid phase doping, etc.N-type dopants may be selected from a group of phosphorus (P), arsenic(As) and antimony (Sb), and p-type dopants may be selected from a groupof boron (B), boron fluoride (BF₂), gallium (Ga), indium (In), andthallium (Tl). The bottom source/drain region 110 may also be formed byan epitaxial growth process. In some embodiments, the epitaxy processcomprises in-situ doping (dopants are incorporated in epitaxy materialduring epitaxy). Epitaxial materials may be grown from gaseous or liquidprecursors. Epitaxial materials may be grown using vapor-phase epitaxy(VPE), molecular-beam epitaxy (MBE), liquid-phase epitaxy (LPE), rapidthermal chemical vapor deposition (RTCVD), metal organic chemical vapordeposition (MOCVD), ultra-high vacuum chemical vapor deposition(UHVCVD), low-pressure chemical vapor deposition (LPCVD), limitedreaction processing CVD (LRPCVD), or other suitable processes. Epitaxialsilicon, silicon germanium (SiGe), germanium (Ge), and/or carbon dopedsilicon (Si:C) silicon can be doped during deposition (in-situ doped) byadding dopants, such as n-type dopants (e.g., phosphorus or arsenic) orp-type dopants (e.g., boron or gallium), depending on the type oftransistor. The dopant concentration can range from 1×10¹⁹ cm⁻³ to3×10²¹ cm⁻³, or preferably between 2×10²⁰ cm⁻³ to 3×10²¹ cm⁻³.

The bottom/source drain regions 114 are surrounded by STI regions orlayer 116. The STI layer 116 may have a height or vertical thickness (indirection Y-Y′) in the range of 50 to 400 nm. The STI layer 116 may beformed from any suitable isolating material.

FIG. 7 shows a cross-sectional view 700 of the FIG. 6 structurefollowing formation of a bottom spacer 118 for the lower VTFETs andfollowing deposition of gate stack materials including a gate dielectriclayer 120 and a gate conductor layer 122 in a gate-first processingscheme.

The bottom spacer 118 for the lower VTFETs is formed surrounding part ofthe bottom portion of the vertical fins 201 above the bottomsource/drain regions 114 and STI layer 116. The bottom spacer 118 may beformed using various processing, such as non-conformal deposition andetch-back processing (e.g., physical vapor deposition (PVD), highdensity plasma (HDP) deposition, etc.). The bottom spacer 118 may beformed of a dielectric material such as SiO₂, SiN, silicon carbide oxide(SiCO), SiBCN, etc. The bottom spacer 118 may have a height or verticalthickness (in direction Y-Y′) in the range of 3 to 10 nm.

After formation of the bottom spacer 118, gate stack materials includingthe gate dielectric layer 120 and the gate conductor layer 122 aredeposited. The gate dielectric layer 120 may be formed of a high-kdielectric material. Examples of high-k materials include but are notlimited to metal oxides such as HfO₂, hafnium silicon oxide (Hf—Si—O),hafnium silicon oxynitride (HfSiON), lanthanum oxide (La₂O₃), lanthanumaluminum oxide (LaAlO₃), zirconium oxide (ZrO₂), zirconium siliconoxide, zirconium silicon oxynitride, tantalum oxide (Ta₂O₅), titaniumoxide (TiO₂), barium strontium titanium oxide, barium titanium oxide,strontium titanium oxide, yttrium oxide (Y₂O₃), aluminum oxide (Al₂O₃),lead scandium tantalum oxide, and lead zinc niobate. The high-k materialmay further include dopants such as lanthanum (La), aluminum (Al), andmagnesium (Mg). The gate dielectric layer 120 may have a uniformthickness in the range of 1 nm to 3 nm.

The gate conductor layer 122 may include a metal gate or work functionmetal (WFM). In some embodiments, the gate conductor layer 122 is formedusing ALD or another suitable process. For nFET devices, the WFM for thegate conductor may be titanium (Ti), aluminum (Al), titanium aluminum(TiAl), titanium aluminum carbon (TiAlC), a combination of Ti and Alalloys, a stack which includes a barrier layer (e.g., of titaniumnitride (TiN) or another suitable material) followed by one or more ofthe aforementioned WFM materials, etc. For pFET devices, the WFM for thegate conductor may be TiN, tantalum nitride (TaN), or another suitablematerial. In some embodiments, the pFET WFM may include a metal stack,where a thicker barrier layer (e.g., of TiN, TaN, etc.) is formedfollowed by a WFM such as Ti, Al, TiAl, TiAlC, or any combination of Tiand Al alloys. It should be appreciated that various other materials maybe used for the gate conductor as desired. The gate conductor layer 122may have a thickness in the range of 5 to 20 nm.

As noted above, illustrative embodiments provide techniques which enablea same gate conductor material (e.g., a same WFM such as TiN) to be usedfor both the lower and upper VTFETs. The gate conductor layer 122 forthe lower VTFETs is annealed as described below, while the gateconductor layer 148 for the upper VTFETs (described in further detailbelow in connection with FIG. 19) is un-annealed (e.g., the gateconductor layer 148 is subject to a laser anneal only, not a RTA).

The FIG. 7 structure may undergo additional processing for forming thegate structure for the lower VTFETs. Such processing may include,following deposition of the gate dielectric layer 120 and the gateconductor layer 122, deposition of an amorphous silicon (a-Si) layer, aRTA reliability anneal, and removal of the a-Si layer.

FIG. 8 shows a cross-sectional view 800 of the FIG. 7 structurefollowing a gate cut. The gate cut, as illustrated, etches portions ofthe gate dielectric layer 120 and the gate conductor layer 122. The gatecut may utilize an etching that removes the material of the gatedielectric layer 120 and the gate conductor layer 122 uniformly, suchthat the gate dielectric layer 120 and gate conductor layer 122 formedover the top surface of the HM 108 are removed, along with portions ofthe gate dielectric layer 120 and the gate conductor layer 122 that areformed between the vertical fins 201 as illustrated.

FIG. 9 shows a cross-sectional view 900 of the FIG. 8 structurefollowing formation of an interlayer dielectric (ILD) layer 124, andfollowing recess of the gate stack materials. The material of the ILDlayer 124 may initially be formed to fill the structure (e.g., withformation of a liner first, where the liner may be SiN), followed bychemical mechanical planarization (CMP) and etch-back. Alternatively,the material of the ILD layer 124 may be formed using HDP and etch-backprocessing to result in the ILD layer 124 as shown in FIG. 9. The ILDlayer 124 may be formed of any suitable isolation material, includingbut not limited to SiO₂, SiOC, SiON, etc.

After formation of the ILD layer 124, the gate stack materials (e.g.,gate dielectric layer 120 and gate conductor layer 122) are recessed,such that the gate stack materials have a height that matches that ofthe ILD layer 124. The ILD layer 124 may have a height or verticalthickness (in direction Y-Y′) in the range of 10 to 30 nm. The recessedgate stack materials provide the gate for the lower VTFETs, which areassumed in this example to be nFET devices as noted above.

FIG. 10 shows a cross-sectional view 1000 of the FIG. 9 structurefollowing formation of a top spacer 126 for the lower VTFETs, andfollowing formation of top source/drain regions 128 for the lowerVTFETs.

The top spacer 126 for the lower VTFETs is formed surrounding part ofthe bottom portion of the vertical fins 201 above the ILD layer 124. Thetop spacer 124 may be formed of similar materials and with similarsizing as that described above with respect to bottom spacer 118.

The top source/drain regions 128 for the lower VTFETs are formed overthe top spacer 126 and surround a remaining part of the bottom portionof the vertical fins 201. The top source/drain regions 128 may be formedof similar materials and with similar processing as that described abovewith respect to bottom source/drain regions 114. The top source/drainregions 128 may have a height or vertical thickness (in direction Y-Y′)in the range of 10 to 30 nm, and may have a width or horizontalthickness (in direction X-X′) in the range of 5 to 15 nm.

FIG. 11 shows a cross-sectional view 1100 of the FIG. 10 structurefollowing formation of a sacrificial oxide layer 130 and followingdeposition and patterning of an organic planarization layer (OPL) 132 asillustrated. The sacrificial oxide layer 130 may be filled over thestructure, followed by recess below a top surface of the topsource/drain regions 128. The sacrificial oxide layer 130 is utilized inthe formation of contacts to the top source/drain regions 128 of thelower VTFETs as will be described in further detail below. Thesacrificial oxide layer 130 may be formed of any suitable oxide, such assilicon oxide (SiOx).

The OPL 132 is deposited and patterned as shown, such that portions ofthe sacrificial oxide layer 130 may be removed to facilitate formationof contacts to desired ones of the top source/drain regions 128 duringmiddle-of-line (MOL) contact formation. In some embodiments, thesacrificial oxide layer 130 is removed during later processing to form acontact to the “left” top source/drain region 128. This may be useful ifthe resulting VTFET structure provides a NAND gate, where a horizontalcontact tunnel extending from the left top source/drain region 128 andconnecting to a vertical via with horizontal contact tunnels to bottomsource/drain regions 140 (formed during downstream processing describedin further detail below) of the upper VTFETs. This shared contact to theleft top source/drain region 128 for one of the lower VTFETs and thebottom source/drain regions 140 of the upper VTFETs can be used as theoutput node for a NAND gate. It should be appreciated, however, thatthis is just one example of MOL contact formation, and that in otherembodiments the sacrificial oxide layer 130 may be suitably patterned(or omitted) as desired to form a particular MOL contact arrangement.

FIG. 12 shows a cross-sectional view 1200 of the FIG. 11 structurefollowing removal of portions of the sacrificial oxide layer 130 exposedby the patterned OPL 132, removal of the OPL 132, and formation ofisolating layer 134. The exposed portions of the sacrificial oxide layer130 may be removed using an etching process that removes the oxidematerial of the sacrificial oxide layer 130 selective to the material ofthe HM 108 and liner 110.

The isolating layer 134, also referred to herein as an N-P isolationlayer 134 or ILD layer 134, is formed to provide isolation between theupper and lower VTFETs. The ILD layer 134 may be formed of a nitridematerial, such as SiN, SiON, etc. The ILD layer 134 may have a height orvertical thickness (in direction Y-Y′) in the range of 20 to 40 nm.

FIG. 13 shows a cross-sectional view 1300 of the FIG. 12 structurefollowing removal of portions of the protection liner 110, and followingdeposition of an oxide layer 136. The protection liner 110 may beremoved using selective ME, wet etching or another suitable process.This may remove portions of the protection liner 110 that are above theILD layer 134, and may extend to form small divots below the ILD layer134 that are later filled with the oxide layer 136. The oxide layer 136may be formed using any suitable deposition (and possible etch-back)processing. The oxide layer 136 has a height that controls the height ofthe bottom source/drain regions 140 formed as described below.

FIG. 14 shows a cross-sectional view 1400 of the FIG. 13 structurefollowing formation of an additional protection liner 138 that protectsthe top portion of the vertical fins 201 above where the bottomsource/drain regions 140 will be formed as described below with respectto FIG. 15. The protection liner 138 may be formed using processing andwith materials similar to that of the protection liner 110. Theprotection liner 138 may have a thickness in the range of 3 nm to 5 nm.

FIG. 15 shows a cross-sectional view 1500 of the FIG. 14 structurefollowing removal of the oxide layer 136 and formation of bottomsource/drain regions 140 for the upper VTFETs. The bottom source/drainregions 140 may be formed using processing similar to that of the bottomsource/drain regions 114. The bottom source/drain regions 140 may have aheight or vertical thickness (in direction Y-Y′) in the range of 10 to30 nm, and may have a width or horizontal thickness (in direction X-X′)in the range of 5 to 15 nm.

FIG. 16 shows a cross-sectional view 1600 of the FIG. 15 structurefollowing formation of a sacrificial oxide layer 142. The sacrificialoxide layer 142 may be used to form contacts to the bottom source/drainregions 140 of the upper VTFETs. As described above, a vertical via andhorizontal tunnels may be formed to provide a shared contact to thebottom source/drain regions 140 of the upper VTFETs and the topsource/drain region of the “left” bottom VTFET to provide an output nodefor a NAND gate. It should be appreciated, however, that this is merelyan example of MOL wiring for a stacked VTFET structure, and thatembodiments are not limited to forming NAND gates using stacked VTFETstructures.

The sacrificial oxide layer 142 may be patterned, as the contact to thebottom source/drain regions 140 may be limited to only a portion of thebottom source/drain regions 140. The particular way in which thesacrificial oxide layer 142 is patterned will vary based on the type ofstructure that is formed using the stacked VTFETs. The sacrificial oxidelayer 142 may initially be formed over the entire structure, followed bypatterning a mask over the sacrificial oxide layer 142 and removal ofexposed portions of the sacrificial oxide layer 142 such that thesacrificial oxide layer remains only in the regions necessary forforming appropriate contacts in downstream processing. FIGS. 17A through17E show a set of cross-sectional views illustrating contact formationfor a NAND gate formed using the stacked VTFET structure.

FIG. 17A shows a first “top-down” cross-sectional view 1700 that istaken across the upper VTFETs of a stacked VTFET structure (e.g., whichshows a layout of the upper VTFETs), which are assumed in this exampleto be pFETs. FIG. 17B shows a second “top-down” cross-sectional view1750 that is taken across the lower VTFETs of the stacked VTFETstructure (e.g., which shows a layout of the lower VTFETs), which areassumed in this example to be nFETs. FIG. 17C shows a third “side”cross-sectional view 1775 that is taken along the line A-A′ of FIGS. 17Aand 17B. FIG. 17D shows a fourth “side” cross-sectional view 1785 thatis taken along the line B-B′ of FIGS. 17A and 17B. FIG. 17E shows afifth “side” cross-sectional view 1795 that is taken along the line C-C′of FIGS. 17A and 17B. Similar numbering in FIGS. 17A through 17E is usedto denote similar elements in the FIG. 16 structure—substrate 1702providing the bottom portion of the fins is similar to substrate 102,isolation layer 1704 is similar to isolation layer 104, semiconductorlayer 1706 providing the upper portion of the fins is similar tosemiconductor layer 106, and bottom source/drain region 1714 is similarto bottom source/drain region 114. Reference numeral 1721 is used todenote the gate stack for the lower VTFETs and reference numeral 1747 isused to denote the gate stack for the upper VTFETs.

Contact 1758 is provided to the top source/drain region (not shown) forthe “left” lower VTFET. The sacrificial oxide layer 142 may be used topattern and provide for this contact, which may be to a ground (GND)connection for the NAND gate structure of FIGS. 17A through 17E. Thecross-sectional view 1600 of FIG. 16 is thus taken “along” the line A-A′similar to the side cross-sectional view 1775 of FIG. 17C. If thecross-sectional view 1600 of FIG. 16 were taken along line B-B′, thenthe sacrificial oxide layer 142 would connect to the top source/drainregion of the “right” lower VTFET (as well as the bottom source/drainregions for the upper VTFETs) to provide for the contact 1760 as shownin the side cross-sectional view 1785 of FIG. 17D. The contact 1760 mayprovide for an output of the NAND gate. Various other patterning ofsacrificial oxide layer 142 may be used in other regions of the stackedVTFET structure as needed to form appropriate contacts for a NAND gateor for other types of devices (e.g., a NOR logic gate, an inverter,etc.).

Contact 1762 is formed to the top source/drain regions of the upperVTFETs, and may provide a connection to a positive supply voltage (e.g.,VDD) for the NAND gate. Contacts 1764-1 and 1764-2 provide for first andsecond input connections for the NAND gate, and contact the gate stacks1721 and 1747 as illustrated in the side cross-sectional view 1795 ofFIG. 17E.

It should be appreciated that the particular contact arrangement shownin FIGS. 17A through 17E is presented by way of example only, and thatembodiments are not limited to using a stacked VTFET structure forforming a NAND gate. In other embodiments, for example, a stacked VTFETstructure may be used to form a NOR gate. For the NOR gate, the groundcontact is to the bottom source/drain regions of the lower VTFETs, theoutput contact is to the top source/drain regions of the lower VTFETsand the top source/drain region of one of the upper VTFETs, the positivevoltage supply contact is to the bottom source/drain regions of theupper VTFETs and the top source/drain region of the other one of theupper VTFETs, and the first and second inputs are similar to those shownfor the NAND gate. For an inverter, which requires only one verticalfin, the ground contact is to the bottom source/drain region of thelower VTFET, the output contact is to the top source/drain region of thelower VTFET and the bottom source/drain region of the upper VTFET, thepositive voltage supply contact is to the top source/drain region of theupper VTFET, and the input is to the gate stacks of the upper and lowerVTFETs. Various other contact arrangements may be used for other typesof devices formed using stacked VTFET structures.

FIG. 18 shows a cross-sectional view 1800 of the FIG. 16 structure afterpatterning of the sacrificial oxide layer 142, removal of the protectionliner 138, and following formation of a bottom spacer 144 for the upperVTFETs surrounding the bottom source/drain regions 140 of the upperVTFETs. The cross-sectional view 1800 is taken along line A-A′ in thetop-down view 1700 of FIG. 17 (e.g., in a region were the sacrificialoxide layer 142 was previously removed). The bottom spacer 144 may beformed of similar materials as the bottom spacer 118. The bottom spacer144 may have a height or vertical thickness (in direction Y-Y′) in therange of 10 to 30 nm, provided that the bottom spacer 126 must be formedwith a greater height than that of the bottom source/drain regions 140so as to provide a buffer between the bottom source/drain regions 140and the gate stack of the upper VTFETs.

After formation of the bottom spacer 144, the FIG. 18 structure mayundergo dopant drive-in for the bottom source/drain regions 140 of theupper VTFETs (e.g., the pFET bottom source/drain regions) and the topsource/drain regions 128 and bottom source/drain regions 114 of thelower VTFETs (e.g., the nFET top and bottom source/drain regions) usinga RTA anneal. The dopant drive-in for the bottom source/drain regions114, top source/drain regions 128, and bottom source/drain regions 140is advantageously done at the same time using the RTA anneal. The dopantdrive-in for the bottom source/drain regions 114, top source/drainregions 128 and bottom source/drain regions 140 is performed prior toformation of the gate stack for the upper VTFETs, as the gate stack forthe upper VTFETs (e.g., the pFET gate stack) should not be exposed tohigh temperatures (e.g., temperatures in the range of 900 to 1100degrees Celsius (° C.)) that are used during the dopant drive-in RTAanneal.

FIG. 19 shows a cross-sectional view 1900 of the FIG. 18 structurefollowing formation of the gate stack for the upper VTFETs includinggate dielectric layer 146 and gate conductor layer 148, and followingformation of an ILD layer 150 and top spacer 152 for the upper VTFETs.The gate dielectric layer 146 and gate conductor layer 148 of the gatestack for the upper VTFETs may be formed of similar materials, withsimilar sizing and similar processing as that described above withrespect to the gate dielectric layer 120 and gate conductor layer 122 ofthe gate stack for the lower VTFETs. More particularly, the gateconductor layer 148 is assumed to be formed of a same material as thegate conductor layer 122 (e.g., the same WFM material such as TiN). Asdescribed above, the gate conductor layer 148 is un-annealed whereas thegate conductor layer 122 is annealed. By un-annealed, it is meant thatthe gate conductor layer 148 is not exposed to a RTA (e.g., a hightemperature anneal). The gate conductor layer 148, as will be describedin further detail below, is exposed to a laser anneal, which has a lowerthermal budget that does not affect the gate stack for the upper VTFETs.

The ILD layer 150 may be formed of similar materials, with similarsizing and similar processing as that described above with respect toILD layer 124. The top spacer 152 for the upper VTFETs may be formed ofsimilar materials, with similar sizing and similar processing as thatdescribed above for the top spacer 126 of the lower VTFETs.

Although not shown, an interfacial layer may be formed between the gatestacks of the upper and lower VTFETs and the sidewalls of the bottom andtop portions of the vertical fins 201 on which the gate stacks areformed. The interfacial layer may be formed of SiO₂ or another suitablematerial such as silicon oxynitride (SiO_(x)N_(y)). The interfaciallayer may have a width or horizontal thickness (in direction X-X′)ranging from 0.5 nm to 1.5 nm.

In some embodiments, the gate stack for the upper VTFETs may be formedas follows. First, the gate dielectric layer 148 may be deposited,followed by deposition of a capping layer (e.g., of TiN) and depositionof an a-Si layer. The structure is then exposed to a reliability anneal(e.g., a 950° C. RTA or laser anneal), after which the a-Si layer andthe capping layer are removed. The gate conductor layer 150 (e.g., whichmay be formed of a pFET WFM such as TiN) is then deposited.

FIG. 20 shows a cross-sectional view 2000 of the FIG. 19 structurefollowing removal of the HM layer 108, recess of the top of the verticalfins 201, and dopant drive-in 2001 for a top junction of the upperVTFETs. The HM layer 108 is removed using selective wet etching. Thevertical fins 201 are then recessed such that a top surface of thevertical fins 201 matches the top surface of the top spacer 152. Thedopant drive-in 2001 includes ion implantation followed by a laser spikeanneal (LSA) to set the top junction for the upper VTFETs. A RTA is notused to set the top junction to protect the gate stack of the upperVTFETs, as the upper VTFETs are assumed to be pFETs and the gate stackfor the pFETs would be damaged by a RTA. It should be noted that,without the dopant drive-in 2001, there is a significant resistancepenalty.

FIG. 21 shows a cross-sectional view 2100 of the FIG. 20 structurefollowing formation of top source/drain regions 154 for the upperVTFETs. The top source/drain regions 154 are formed over the topsurfaces of the vertical fins 201. The top source/drain regions 154 maybe formed of similar materials and using similar processing as thatdescribed above with respect to bottom source/drain regions 114. The topsource/drain regions 154 may have a height or vertical thickness (indirection Y-Y′) in the range of 10 to 30 nm, and may have a width orhorizontal thickness (in direction X-X′) in the range of 10 to 30 nm.After formation of the top source/drain regions 154, a short laseranneal (e.g., with a duration in the range of 10 nanoseconds to 10milliseconds at a temperature of about 1200° C.) is performed.Advantageously, the laser anneal is very short and has a thermal budgetthat avoids damage to the gate stack of the upper VTFETs. An ILD layer156 may then be formed over the top source/drain regions 154, followedby additional processing to form contacts for the upper and lowerVTFETs. The ILD layer 156 may be formed of similar materials as the ILDlayer 124. The ILD layer 156, as shown in FIG. 21, overfills thestructure, and may have a height or vertical thickness (in directionY-Y′) that exceeds the top surfaces of the top source/drain regions 154,such as a height or vertical thickness in the range of 30 to 70 nm.

In some embodiments, a method of forming a semiconductor structurecomprises forming one or more vertical fins each comprising a firstsemiconductor layer providing a vertical transport channel for a lowerVTFET of a stacked VTFET structure, an isolation layer over the firstsemiconductor layer, and a second semiconductor layer over the isolationlayer providing a vertical transport channel for an upper VTFET of thestacked VTFET structure. The method also comprises forming a first gatestack comprising a first gate dielectric layer and a first gateconductor layer surrounding a portion of the first semiconductor layerof each of the one or more vertical fins. The method further comprisesforming a second gate stack comprising a second gate dielectric layerand a second gate conductor layer surrounding a portion of the secondsemiconductor layer of each of the one or more vertical fins. The firstgate conductor layer and the second gate conductor layer comprise a samematerial.

Forming the first gate stack may comprise utilizing a gate-first processand forming the second gate stack may comprise utilizing a gate-lastprocess.

The first gate conductor layer may be annealed and the second gateconductor layer may be un-annealed. The lower VTFET may comprise an nFETand the upper VTFET may comprise a pFET.

The first gate conductor layer may comprise a given WFM that is annealedand the second gate conductor layer comprises the given WFM that isun-annealed. The given WFM may comprise TiN.

Forming the first gate stack may comprise forming the first gatedielectric layer over the one or more vertical fins and a first bottomspacer surrounding a portion of the first semiconductor layer of the oneor more vertical fins, forming the first gate conductor layer over thefirst gate dielectric layer, forming an amorphous silicon layer over thefirst gate conductor layer, performing a reliability anneal, removingthe amorphous silicon layer, performing a gate cut etch to removeportions of the first gate dielectric layer and the first gate conductorlayer formed over portions of the first bottom spacer spaced apart fromsidewalls of the one or more vertical fins, forming an interlayerdielectric layer over the first bottom spacer surrounding a portion ofthe first gate dielectric layer and the first gate conductor layer, andrecessing the first gate dielectric layer and the first gate conductorlayer to a top surface of the interlayer dielectric layer to provide thefirst gate stack.

Forming the second gate stack may comprise forming the second gatedielectric layer over the one or more vertical fins and a second bottomspacer surrounding a portion of the second semiconductor layer of theone or more vertical fins, forming a capping layer over the second gatedielectric layer, forming an amorphous silicon layer over the cappinglayer, performing a reliability anneal, removing the amorphous siliconlayer and the capping layer, forming the second gate conductor layerover the second gate dielectric layer, performing a gate cut etch toremove portions of the second gate dielectric layer and the second gateconductor layer formed over portions of the second bottom spacer spacedapart from sidewalls of the one or more vertical fins, forming aninterlayer dielectric layer over the second bottom spacer surrounding aportion of the second gate dielectric layer and the second gateconductor layer, and recessing the second gate dielectric layer and thesecond gate conductor layer to a top surface of the interlayerdielectric layer to provide the second gate stack.

The method may further comprise performing a RTA to provide dopantdrive-in at the same time for (i) first bottom source/drain regions ofthe lower VTFET, (ii) first top source/drain regions of the lower VTFETand (iii) second bottom source/drain regions of the upper VTFET.

The method may further comprise patterning a hard mask layer over a topsurface of the second semiconductor layer, etching the secondsemiconductor layer and the isolation layer exposed by the patternedhard mask layer to form a first portion of the one or more verticalfins, forming a first liner on sidewalls of the first portion of the oneor more vertical fins, etching exposed portions of a substrate below theisolation layer to provide a first portion of the first semiconductorlayer of the one or more vertical fins, forming a second liner onsidewalls of the first portion of the first semiconductor layer of theone or more vertical fins and on sidewalls of the first liner, etchingexposed portions of the substrate below the second liner to provide asecond portion of the first semiconductor layer of the one or morevertical fins, and trimming sidewalls of the second portion of the firstsemiconductor layer of the one or more vertical fins to match sidewallsof the first portion of the first semiconductor layer of the one or morevertical fins.

The method may further comprise forming first bottom source/drainregions over a top surface of the substrate and surrounding the secondportion of the first semiconductor layer of the one or more verticalfins, removing the second liner, patterning the first bottomsource/drain regions, forming shallow trench isolation regionssurrounding the first bottom source/drain regions, and forming a firstbottom spacer over the first bottom source/drain regions and the shallowtrench isolation regions.

Forming the first gate stack may comprise forming the first gatedielectric layer over the one or more vertical fins and the first bottomspacer, forming the first gate conductor layer over the first gatedielectric layer, forming an amorphous silicon layer over the first gateconductor layer, performing a reliability anneal, removing the amorphoussilicon layer, performing a gate cut etch to remove portions of thefirst gate dielectric layer and the first gate conductor layer formedover the hard mask layer and over portions of the first bottom spacer,forming a first interlayer dielectric layer over the first bottom spacersurrounding a portion of the first gate dielectric layer and the firstgate conductor layer, and recessing the first gate dielectric layer andthe first gate conductor layer to a top surface of the first interlayerdielectric layer to provide the first gate stack.

The method may further comprise forming a first top spacer over thefirst gate stack and the first interlayer dielectric layer, formingfirst top source/drain regions surrounding a remainder of the secondportion of the first semiconductor layer over the first top spacer,forming a second interlayer dielectric layer surrounding the first topsource/drain regions, the isolation layer and a first portion of thesecond semiconductor layer of the one or more vertical fins, removingthe first liner, forming an oxide layer over the second interlayerdielectric layer, forming a third liner on exposed sidewalls of thesecond semiconductor layer and the hard mask layer of the one or morevertical fins, removing the oxide layer, forming second bottomsource/drain regions surrounding exposed sidewalls of the secondsemiconductor layer below the third liner, forming a second bottomspacer surrounding the second bottom source/drain regions, andperforming a RTA to provide dopant drive-in for the first bottomsource/drain regions, the first top source/drain regions and the secondbottom source/drain regions.

Forming the second gate stack may comprise forming the second gatedielectric layer over the one or more vertical fins and the secondbottom spacer, forming a capping layer over the second gate dielectriclayer, forming an additional amorphous silicon layer over the cappinglayer, performing an additional reliability anneal, removing theadditional amorphous silicon layer and the capping layer, forming thesecond gate conductor layer over the second gate dielectric layer,performing an additional gate cut etch to remove portions of the secondgate dielectric layer and the second gate conductor layer formed overthe hard mask layer and over portions of the second bottom spacer,forming a third interlayer dielectric layer over the second bottomspacer surrounding a portion of the second gate dielectric layer and thesecond gate conductor layer, and recessing the second gate dielectriclayer and the second gate conductor layer to a top surface of the thirdinterlayer dielectric layer to provide the second gate stack.

The method may further comprise removing the hard mask layer, recessinga top surface of the second semiconductor layer to match a top surfaceof the third interlayer dielectric layer, performing ion implantationand a laser spike anneal to form a top junction in the top surface ofthe second semiconductor layer of the one or more vertical fins, formingsecond top source/drain regions over the top surface of the secondsemiconductor layer of the one or more vertical fins, and performing alaser anneal.

In some embodiments, a semiconductor structure comprises one or morevertical fins each comprising a first semiconductor layer providing avertical transport channel for a lower VTFET of a stacked VTFETstructure, an isolation layer over the first semiconductor layer, and asecond semiconductor layer over the isolation layer providing a verticaltransport channel for an upper VTFET of the stacked VTFET structure. Thesemiconductor structure also comprises a first gate stack comprising afirst gate dielectric layer and a first gate conductor layer surroundinga portion of the first semiconductor layer of each of the one or morevertical fins. The semiconductor structure further comprises a secondgate stack comprising a second gate dielectric layer and a second gateconductor layer surrounding a portion of the second semiconductor layerof each of the one or more vertical fins. The first gate conductor layerand the second gate conductor layer comprise a same material.

The first gate conductor layer may be annealed and the second gateconductor layer may be un-annealed. The lower VTFET may comprise an nFETand the upper VTFET may comprise a pFET.

The first gate conductor layer may comprise a given WFM that is annealedand the second gate conductor layer may comprise the given WFM that isun-annealed. The given WFM may comprise TiN.

In some embodiments, an integrated circuit comprises a stacked VTFETstructure. The stacked VTFET structure comprises one or more verticalfins each comprising a first semiconductor layer providing a verticaltransport channel for a lower VTFET of the stacked VTFET structure, anisolation layer over the first semiconductor layer, and a secondsemiconductor layer over the isolation layer providing a verticaltransport channel for an upper VTFET of the stacked VTFET structure. Thestacked VTFET structure also comprises a first gate stack comprising afirst gate dielectric layer and a first gate conductor layer surroundinga portion of the first semiconductor layer of each of the one or morevertical fins. The stacked VTFET structure further comprises a secondgate stack comprising a second gate dielectric layer and a second gateconductor layer surrounding a portion of the second semiconductor layerof each of the one or more vertical fins. The first gate conductor layerand the second gate conductor layer comprise a same material.

The first gate conductor layer may be annealed and the second gateconductor layer may be un-annealed. The lower VTFET may comprise an nFETand the upper VTFET may comprise a pFET.

The first gate conductor layer may comprise a given WFM that is annealedand the second gate conductor layer may comprise the given WFM that isun-annealed. The given WFM may comprise TiN.

It is to be appreciated that the various materials, processing methods(e.g., etch types, deposition types, etc.) and dimensions provided inthe discussion above are presented by way of example only. Various othersuitable materials, processing methods, and dimensions may be used asdesired.

Semiconductor devices and methods for forming same in accordance withthe above-described techniques can be employed in various applications,hardware, and/or electronic systems. Suitable hardware and systems forimplementing embodiments of the invention may include, but are notlimited to, sensors an sensing devices, personal computers,communication networks, electronic commerce systems, portablecommunications devices (e.g., cell and smart phones), solid-state mediastorage devices, functional circuitry, etc. Systems and hardwareincorporating the semiconductor devices are contemplated embodiments ofthe invention. Given the teachings provided herein, one of ordinaryskill in the art will be able to contemplate other implementations andapplications of embodiments of the invention.

Various structures described above may be implemented in integratedcircuits. The resulting integrated circuit chips can be distributed bythe fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of forming a semiconductor structure,comprising: forming one or more vertical fins each comprising a firstsemiconductor layer providing a vertical transport channel for a lowervertical transport field-effect transistor of a stacked verticaltransport field-effect transistor structure, an isolation layer over thefirst semiconductor layer, and a second semiconductor layer over theisolation layer providing a vertical transport channel for an uppervertical transport field-effect transistor of the stacked verticaltransport field-effect transistor structure; forming a first gate stackcomprising a first gate dielectric layer and a first gate conductorlayer surrounding a portion of the first semiconductor layer of each ofthe one or more vertical fins; and forming a second gate stackcomprising a second gate dielectric layer and a second gate conductorlayer surrounding a portion of the second semiconductor layer of each ofthe one or more vertical fins; wherein the first gate conductor layerand the second gate conductor layer comprise a same material.
 2. Themethod of claim 1, wherein forming the first gate stack comprisesutilizing a gate-first process and wherein forming the second gate stackcomprises utilizing a gate-last process.
 3. The method of claim 1,wherein the first gate conductor layer is annealed and the second gateconductor layer is un-annealed.
 4. The method of claim 3, wherein thelower vertical transport field-effect transistor comprises an n-typefield-effect transistor and the upper vertical transport field-effecttransistor comprises a p-type field-effect transistor.
 5. The method ofclaim 1, wherein the first gate conductor layer comprises a given workfunction metal that is annealed and the second gate conductor layercomprises the given work function metal that is un-annealed.
 6. Themethod of claim 5, wherein the given work function metal comprisestitanium nitride (TiN).
 7. The method of claim 1, further comprisingforming first bottom source/drain regions for the lower verticaltransport field-effect transistor structure surrounding a portion of thefirst semiconductor layer of each of the one or more vertical fins. 8.The method of claim 1, further comprising forming first top source/drainregions for the lower vertical transport field-effect transistorstructure surrounding a portion of the first semiconductor layer of eachof the one or more vertical fins.
 9. The method of claim 1, furthercomprising forming second bottom source/drain regions for the uppervertical transport field-effect transistor structure surrounding aportion of the second semiconductor layer of each of the one or morevertical fins.
 10. The method of claim 1, further comprising formingsecond top source/drain regions for the upper vertical transportfield-effect transistor structure surrounding a portion of the secondsemiconductor layer of each of the one or more vertical fins.
 11. Asemiconductor structure, comprising: one or more vertical fins eachcomprising a first semiconductor layer providing a vertical transportchannel for a lower vertical transport field-effect transistor of astacked vertical transport field-effect transistor structure, anisolation layer over the first semiconductor layer, and a secondsemiconductor layer over the isolation layer providing a verticaltransport channel for an upper vertical transport field-effecttransistor of the stacked vertical transport field-effect transistorstructure; a first gate stack comprising a first gate dielectric layerand a first gate conductor layer surrounding a portion of the firstsemiconductor layer of each of the one or more vertical fins; and asecond gate stack comprising a second gate dielectric layer and a secondgate conductor layer surrounding a portion of the second semiconductorlayer of each of the one or more vertical fins; wherein the first gateconductor layer and the second gate conductor layer comprise a samematerial.
 12. The semiconductor structure of claim 11, wherein the firstgate conductor layer is annealed and the second gate conductor layer isun-annealed.
 13. The semiconductor structure of claim 12, wherein thelower vertical transport field-effect transistor comprises an n-typefield-effect transistor and the upper vertical transport field-effecttransistor comprises a p-type field-effect transistor.
 14. Thesemiconductor structure of claim 11, wherein the first gate conductorlayer comprises a given work function metal that is annealed and thesecond gate conductor layer comprises the given work function metal thatis un-annealed.
 15. The semiconductor structure of claim 14, wherein thegiven work function metal comprises titanium nitride (TiN).
 16. Anintegrated circuit comprising: a stacked vertical transport field-effecttransistor structure comprising: one or more vertical fins eachcomprising a first semiconductor layer providing a vertical transportchannel for a lower vertical transport field-effect transistor of thestacked vertical transport field-effect transistor structure, anisolation layer over the first semiconductor layer, and a secondsemiconductor layer over the isolation layer providing a verticaltransport channel for an upper vertical transport field-effecttransistor of the stacked vertical transport field-effect transistorstructure; a first gate stack comprising a first gate dielectric layerand a first gate conductor layer surrounding a portion of the firstsemiconductor layer of each of the one or more vertical fins; and asecond gate stack comprising a second gate dielectric layer and a secondgate conductor layer surrounding a portion of the second semiconductorlayer of each of the one or more vertical fins; wherein the first gateconductor layer and the second gate conductor layer comprise a samematerial.
 17. The integrated circuit of claim 16, wherein the first gateconductor layer is annealed and the second gate conductor layer isun-annealed.
 18. The integrated circuit of claim 17, wherein the lowervertical transport field-effect transistor comprises an n-typefield-effect transistor and the upper vertical transport field-effecttransistor comprises a p-type field-effect transistor.
 19. Theintegrated circuit of claim 16, wherein the first gate conductor layercomprises a given work function metal that is annealed and the secondgate conductor layer comprises the given work function metal that isun-annealed.
 20. The integrated circuit of claim 19, wherein the givenwork function metal comprises titanium nitride (TiN).